(1) Field of the Invention
The present invention relates to a long-range aircraft having a high forward speed in cruising flight.
This advanced rotorcraft concept seeks to combine at reasonable cost the effectiveness in vertical flight of a conventional helicopter with the high travel speed performance made possible by using propulsion propellers and installing modern engines.
(2) Description of Related Art
In order to understand the object of the invention clearly, it is appropriate to recall the main types of flying machine corresponding to airplanes and to rotorcraft.
The term “rotorcraft” designates any aircraft in which lift is provided in full or in part by at least one rotary wing. The rotary wing usually comprises at least one rotor of large diameter and of axis that is substantially vertical while the aircraft is standing on the ground.
The rotorcraft category includes several distinct types of aircraft.
Firstly, there is the helicopter having at least one main rotor that is driven by a suitable power plant and that provides both lift and propulsion.
A helicopter may have two lift rotors providing it with lift and propulsion. These two rotors may be arranged one behind the other along the longitudinal axis of said rotorcraft: the aircraft is then referred to as a tandem-rotor rotorcraft.
In a tandem-rotor rotorcraft, the first and second lift rotors are generally contrarotating so that the yaw torque generated in one direction by the first rotor is greatly reduced or even eliminated by the yaw torque generated in the opposite direction by the second rotor. The combined effect of the yaw torque from both rotors thus enables the rotorcraft to be stabilized relative to its yaw axis in straight-line flight, without wind.
It should be observed that it is possible to synchronize the speeds of rotation of the rotors in order to guarantee that the blades of the rotors do not strike one another. It is then said that the blades are “meshing” since the rotary blades of the first rotor present a constant azimuth phase offset relative to the rotating blades of the second rotor.
Also known is the autogyro, which is a rotorcraft in which the rotor does not receive power, but provides lift by rotating in autorotation under the effect of the forward speed of the aircraft.
There is also the gyrodyne, which is a rotorcraft intermediate between the helicopter and the autogyro, in which the rotor provides only lift. The rotor is normally driven by a power plant during stages of takeoff, hovering flight, vertical flight, and landing, like a helicopter. A gyrodyne also has an additional propulsion system that is essentially different from the rotor assembly. In forward flight, the rotor continues to provide lift, but only in autorotation mode, i.e. without power being transmitted to said rotor.
Also known is the compound rotorcraft that takes off and lands like a helicopter, and that performs cruising flight like an autogyro.
Furthermore, the convertible rotorcraft constitutes another particular rotorcraft formula. This term covers all rotorcraft that change configuration while in flight: takeoff and landing in a helicopter configuration, cruising flight in an airplane configuration, with two rotors being tilted through about 90 degrees, for example, in order to act as propellers.
Of these various rotorcraft formulae, the helicopter is the simplest, and as such it has become the most common in spite of the fact that the maximum forward speed of a helicopter is about 300 kilometers per hour (km/h), which is small, and less than the speed that can be envisaged with formulae of the compound or convertible types, given that they are technically more complex and more expensive.
There is another, novel formula that is known and that goes by the name “hybrid helicopter” for convenience.
A hybrid helicopter has a fuselage and a main rotor for driving blades in rotation under power from at least one engine, advantageously two turbine engines arranged on top of the fuselage on either side of the longitudinal plane of symmetry of the aircraft. The hybrid helicopter is also provided with a wing and with at least one propulsion propeller.
Furthermore, the hybrid helicopter is fitted with an integrated drive train that comprises not only the engine, the rotor, and the propeller, but also a mechanical system interconnecting those elements.
With this configuration, the hybrid helicopter is remarkable in that the speeds of rotation of the engine outlets, of the rotor, of the propeller, and of the mechanical interconnection system are mutually proportional, with the proportionality ratio being constant regardless of the flying configuration of the hybrid helicopter under normal conditions of operation of the integrated drive train.
Consequently, and advantageously, the rotor is always driven in rotation by the engine and always develops lift regardless of the configuration of the hybrid helicopter, both in forward flight and in hovering flight. The hybrid helicopter is thus neither an autogyro, nor a gyrodyne, nor a compound rotorcraft, but is a novel type of rotorcraft.
The hybrid helicopter enables missions to be carried out during long periods of time in vertical flight, enables cruising flight to be performed at high speed, and also makes it possible to cover long ranges, while nevertheless being capable of performing hovering flight and taking off vertically.
In the state of the art, mention may be made of documents FR 2 946 315 and FR 2 929 243.
Furthermore, among documents presenting aircraft having two rotors, documents US 2009/0216392 and US 2009/0014580 are known.
Those documents US 2009/0216392 and US 2009/0014580 describe an aircraft that has two rotors in tandem, and that is fitted on either side of its structure with respective swivel-mounted ducted propellers, the ducting of each propeller being also fitted with a vane for redirecting the exhaust stream from the engine.
Document GB 1 120 658 describes an aircraft having two rotors in tandem and engines capable of providing thrust by delivering power simultaneously on an outlet shaft.
Each engine forms part of an engine-propulsion group including a low pressure turbine setting a propeller into rotation. Furthermore, each engine includes a free turbine either for driving the two rotors in tandem, or else for contributing to driving the propeller of the engine-propulsion group via clutches.
Document U.S. Pat. No. 3,905,565 describes a piloting system for an aircraft fitted with two rotors in tandem.
Document WO 2007/014531 describes an aircraft having two superposed contrarotating rotors, together with two tilting propellers.
Documents US 2006/269414, U.S. Pat. No. 7,296,767, and U.S. Pat. No. 7,628,355 describe an aircraft having two superposed contrarotating rotors that are associated with a propulsion propeller.